The present invention relates to a new and improved offset printing apparatus and method in which an image is transferred from a plate cylinder to a blanket cylinder during printing.
Known printing presses include a plate cylinder upon which a printing plate is mounted. During a printing operation, an image is transferred from the printing plate to a blanket cylinder. The image is then transferred from the blanket cylinder to the material being printed.
Although the printing plate normally extends almost completely around the plate cylinder, there is frequently a small gap at the ends of the printing plate. This gap extends longitudinally along the peripheral surface of the plate cylinder in a direction parallel to the central axis of the plate cylinder. Since the peripheral surfaces of the printing plate and blanket cylinder are in rolling engagement during operation of a printing press, the gap strikes the blanket cylinder repeatedly at the same location. This tends to result in wear, deformation and/or damage to a blanket on the blanket cylinder. Of course, this is detrimental to printing quality.
During operation of a printing press, ink tends to build up on the blanket cylinder at locations where ink is repetitively applied to the blanket cylinder by the printing plate. This build up becomes particularly objectionable when the printing plate has relatively dark or high ink density areas adjacent to light or relatively low ink density areas. In order to maintain the requisite quality of printing, it is necessary to remove the ink build up by periodically washing the blanket cylinders. Of course, this reduces productivity.